Devices incorporating a transmit coil and a receive coil to detect the presence of metal have long been used in the metal detection art. In these devices a transmit coil has an oscillating current passed therethrough, and a receive coil is disposed adjacent the transmit coil such that the presence of metallic objects nearby causes a magnetic coupling between the transmit and receive coils. The receive coil then carries a current oscillating at the same frequency as the transmit coil. In these prior art devices, circuitry is provided to detect such a current in the receive coil or a preselected rate of change of current in the receive coil.
A drawback of prior metal detectors is that the transmit and receive coils are located at a position isolated from the detection circuitry. This isolation has been necessary because, had the receive coil been located at a position near the circuitry, the circuitry would have interacted with the receive coil and either given false detection signals or attenuated the sensitivity of the device by elevating the background level of magnetic coupling.
A further drawback of prior metal detectors is the lack of a convenient low battery voltage warning device. In many devices, it is necessary for the operator to take an affirmative step, such as closing a switch, in order to perform a battery test. In some devices, battery testing is not possible and the operator always takes a chance that the device will fail in the field due to an insufficiently charged battery.